1. Field of the Invention
The present invention relates to determining the level of a fluid in a well such as a gas well, an oil well, or water well.
2. Description of the Art Practices
It is known that wells replenish fluids at different rates even in the same formation or well field. The rate of fluid flow into the well bore is maximized because the hydrostatic head driving the fluid is at a maximum. See for example Burris, et al., U.S. Pat. No. 6,085,836 issued Jul. 11, 2000. The Burris, et al., patent is incorporated herein by reference.
The preceding observation suggests that the well pump should run constantly to keep the level in the well bore as low as possible thus maximizing production. Of course, this is often unsatisfactory for several reasons.
First, running the pump constantly or at too great a speed is inefficient since, some of the time, the well bore is completely empty and there is nothing to pump. Thus, energy conservation becomes a cost consideration. Second, the equipment is subject to wear and damage resulting in costly repairs when pumps are run dry. Third, paraffin build up is more pronounced when a well is allowed to pump dry. In the dry pump condition gases are drawn into the bore. The gases in the bore then expand and cool. As the gases cool, paraffin build up is promoted as these high melting hydrocarbons begin to plate out on the surfaces of the bore. However, a well may be pumped continuously provided that the liquid level of the well is high enough to ensue the well sump has liquid therein, e.g. avoid pumping gas into the tubing.
Given the above considerations, control strategies aimed at optimizing well production have emerged. Notably, timers have been used to control the pump duty cycle. A timer may be programmed to run the well nearly perfectly if the one could determine the duration of the on cycle and off cycle which keeps the fluid level in the bore low but which does not pump the bore dry.
The pump on cycle and off cycle can be determined for a group of wells or for an entire well field. Savings in energy may be maximized by knowing which wells fill at what rate and then optimizing pumping to reduce or maintain a constant electric load below the maximum peak available.
Given fluid level information, deciding when or how fast to run the pump is very straightforward and production can be optimized. Fluid level determinations, particularly for deep down hole (bore) systems, have been implemented. Unfortunately, these deep down hole systems have been costly and complex to install, unreliable in operation, and costly to repair or service. Although the implementation details will not be discussed here, it is worth noting that these systems, when operating correctly, have proven that significant gains in well production are available when control strategies using fluid level measurement are applied.
One system that has been attempted is the use of one-shot measurements. The one-shot measurement will use a sonic event such as a shotgun shell to generate the event. Another system is based on a nitrogen tank being utilized to generate a sonic event. In either of the foregoing systems the production of the well must be shut down to implement the sonic event and the corresponding data evaluations. By contrast the present invention will permit continuous operation of the well as the sonic events are generated, the data collected, the well conditions read out, and changes in pumping implemented. Moreover, the system of the present invention is conducted utilizing fluid from the well thus avoiding the cost of the nitrogen and does not require opening of the well to the atmosphere.
Clearly, what is needed is a control system with the advantages of fluid level measurement which is cost effective to install and operate and which is reliable. Basic features for fluid level measurement should include applicability to oil, water, or other wells and should be applicable to rod, screw (such as by a frequency drive), or other pump types.
A fluid level measurement system should be simple and inexpensive to install in the T-Head and useful for well depths to 10,000 feet. Such a fluid level measurement system should be self calibrating for each installation and accurate to 10 feet (3.1 meters). The system should be robust to harsh environments within and around the well.
A fluid level measurement system is desirably able to provide fluid level measurements in well in which gas is produced under vacuum. That is, some wells do not have sufficient pressure in the well to permit the gas to flow to the T-Head. In such cases, the well is often one in which methane is derived from a coal seam in which progressive cavity pumps are employed.